Electromagnetic interference (EMI) in electronic products such as a home theater system or DVD players is a real issue that must be dealt with in order to meet the maximum allowed regulated level.
There are many techniques to reduce the radiated power and all of them add to the cost of a given product. The most obvious way to reduce the actual electromagnetic energy radiated from a product is to increase the shielding of the box in which the electronic circuits are placed. With this method, the hardware and assembly add significant cost to the product.
A less obvious method that is known in the art is to modulate the system clock of the computing devices such that the radiated power level in a given bandwidth is lower. It is called spread spectrum clock generation (SSCG). This technique is effective and popular since the system clock is one of the major contributors in EMI and the cost to the system is minimal. The legislation governing EMI in products specifies that the radiated power limits per bandwidth unit and the standard bandwidth for this type of measurement is 120 KHz. It is easy to understand the advantages of modulating the system clock in that context, but care must be taken to guarantee adequate operation. For example, it would detrimental to modulate the frequency up from the reference in a digital system with very little margin in setup and hold time. It would be equally bad for a digital system performing a real time application up to its speed limit and modulating the frequency down from the reference. In order to address these issues, SSCG circuits in general can modulate the reference frequency down, up or center. FIG. 3 shows the three types of modulation, Up, Down and Center spread.
The commonly used technique to produce SSCG is an “all analog” one where modulation is applied or inserted into a phased lock loop (PLL). The frequency can be modulated by imposing a signal on the voltage control node of a VCO in a PLL. This method is the most obvious one in a system with a PLL and is proven to work very well for most applications. There are many other techniques to modulate the system clock. For example, one could uses a “fractional N” technique that changes the divider ratio to produce the modulation. All methods found in the literature use some type of analog circuit. Analog circuit, however, have shortcomings, including a long product integration time, and less predictability in performance.
Therefore, there exists a need for an improved system and method of SSCG. As will be seen, the invention provides such a system and method in an elegant manner.